JPH0738615B2 - Optical FSK frequency shift stabilization circuit - Google Patents

Description

The present invention relates to an optical FSK frequency shift for stabilizing the frequency shift of FSK signal light obtained by direct modulation of each laser diode in an optical frequency multiplex transmission system. Regarding the stabilizing circuit.

[Conventional technology]

FIG. 9 is a diagram showing a schematic configuration of an optical FSK modulator that directly modulates the signal light output from the laser diode.

In the figure, a laser diode 91 is configured to output a signal light (FSK signal light) having a frequency according to an injection current,
The FSK signal light 99 whose frequency changes according to the digital signal 95 is obtained by adding the bias current 93 and the digital signal 95, which is the transmission signal, by the adder 97 to obtain the injection current.
Is output.

However, in response to changes in the temperature of the substrate equipped with the laser diode 91 and other laser diode driving conditions, the FS
Since the frequency shift of the K signal light fluctuates, a frequency shift stabilizing circuit that stabilizes the frequency shift has been proposed (Japanese Patent Application No. 63-192337).

In this stabilization method, the FSK signal light is passed through a periodic filter whose transmittance changes periodically according to frequency (hereinafter, the frequency interval in which the transmittance changes cyclically is referred to as "transmittance cycle"), and FSK signal light is transmitted. A signal detected according to the deviation from the transmittance cycle of the periodic filter set corresponding to the frequency shift of the signal light is extracted. That is, FS by the periodic filter
Utilizing the fact that the fluctuation of the frequency shift of the K signal light can be detected, feedback is applied to the FSK signal light source using the detection signal,
This configuration stabilizes the frequency shift of the FSK signal light by controlling the amplitude of the input digital signal or controlling the frequency modulation efficiency using a multi-electrode laser diode.

It should be noted that a configuration in which the FSK signal light is converted into an electric signal and the frequency shift of the FSK signal light is stabilized to the transmittance cycle by using the electric signal level periodic filter is also shown.

[Problems to be Solved by the Invention]

By the way, the frequency shift stabilization circuit already proposed is used for an optical FSK modulator using a single laser diode as an FSK signal light source, and optical frequency multiplex transmission using a plurality of FSK signal light sources. The optical FSK frequency shift stabilization circuit used in the system is not shown.

It is an object of the present invention to provide an optical FSK frequency shift stabilizing circuit that can individually perform feedback control on a plurality of FSK signal light sources and suppress fluctuations in the frequency shift of a plurality of FSK signal lights.

[Means for Solving the Problems]

FIG. 1 is a block diagram showing the principle configuration of the method of the present invention.

In the figure, the frequency shifts according to the transmission signal, and a plurality of light sources that emit a plurality of FSK signal light having different center frequencies, and the FSK whose center frequency is within a predetermined frequency range from the plurality of FSK signal lights. Selection means for selectively extracting signal light, the selected FSK signal light is input, a periodic filter that periodically changes the transmittance at a predetermined frequency interval, and the transmission of the periodic filter according to the output of the periodic filter. The control means for matching the frequency at which the rate has a maximum value or a minimum value and the center frequency of the FSK signal light, and detecting the deviation of the frequency shift amount of the FSK signal light with respect to the half cycle of the frequency interval, and the selection. And switching means for transmitting the output signal of the control means to the injection current control section of the corresponding light source in association with the selection operation of the means.

[Work]

The selecting means extracts one FSK signal light from a plurality of FSK signal lights that are frequency-division multiplexed. In the periodic filter and the control means, the center frequency of the FSK signal light is matched with the frequency at which the transmittance has a maximum value or a minimum value,
By taking out an output signal corresponding to the deviation of the frequency shift amount of the FSK signal light with respect to the half cycle of the transmittance cycle, and returning the output signal to the corresponding light source via the switching means,
It is possible to stabilize the frequency shift of the FSK signal light.

That is, the transmittance of the periodic filter is controlled to have a maximum value or a minimum value at the center frequency of the FSK signal light, and
By using the periodic filter and the control means for stabilizing the frequency shift of the K signal light to the half cycle of the transmittance period, only by adding the selecting means and the switching means, the one periodic filter and the controlling means are used. It is possible to stabilize each frequency shift of a plurality of FSK signal lights.

When the center frequency interval of the optical frequency-multiplexed FSK signal light is an integer multiple of the transmittance cycle of the periodic filter, the frequency characteristics of the transmittance of the periodic filter for each FSK signal light are respectively The injection current controller of each light source can be configured in the same manner.

If the center frequency interval of the optical frequency-multiplexed FSK signal light is a half integer multiple of the transmittance cycle of the periodic filter (an integer multiple of half cycle), the periodic filter for each FSK signal light is used. Since the frequency characteristic of the transmittance is sequentially reversed and the sign of the output signal of the control means is reversed, it is necessary to sequentially set the control characteristic of the injection current control unit of each light source to the reverse characteristic accordingly.

〔Example〕

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 2 is a block diagram showing the configuration of the first embodiment of the optical FSK frequency deviation stabilizing circuit of the present invention.

In the present embodiment, a waveguide type Mach-Zehnder interferometer is used as the periodic filter. The transmittance period of this waveguide type Mach-Zehnder interferometer is set to 4 (GHz), and the center frequency interval of each optical frequency multiplexed FSK signal light is stabilized to 10 (GHz) by a predetermined control means. In the configuration, it is assumed that the frequency shift of each FSK signal light is stabilized to 2 (GHz) which is half of the transmittance cycle.

In the figure, N digital signals 11 to be multiplexed are transmitted.
_{1 to} 11 _N are adders 21 _{1 to} 21 via variable attenuators 20 _{1 to} 20 _N
It is input to _N , added to _N bias currents 13 _{1 to} 13 _N , and injected into _N laser diodes 22 _{1 to} 22 _N. The N FSK signal lights 15 ₁ to 15 _N emitted from the laser diodes 22 _{1 to} 22 _N are passed through the optical multiplexer 23 to generate FSK signals.
Optical frequency multiplexing is performed on the signal light 15.

This optical frequency-multiplexed FSK signal light 15 is passed through the optical frequency selection switch 24 and the FSK signal light 15 _M of one wavelength is transmitted.
(1 ≦ M ≦ N) is selected and is incident on the waveguide type Mach-Zehnder interferometer 25. Waveguide Mach-Zehnder interferometer 25
The light emitted from the two output ports a and b are received by the photodiodes 26 ₁ and 26 ₂ connected in series, respectively. A control signal extracted as a differential signal of the output of each photodiode from the connection point of the photodiodes 26 ₁ and 26 _2.
17 is an optical path length control unit of the waveguide type Mach-Zehnder interferometer 25.
27 and the changeover switch 28 for switching between 1 and N. Each output of the changeover switch 28 are respectively connected to the attenuation control terminal of the variable attenuator 20 ₁ to 20 _N. Control circuit 2
Reference numeral 9 controls the selection operation of the optical frequency selection switch 24 and the switching operation of the changeover switch 28 in association with each other.

When the maximum point or the minimum point of the transmittance of the waveguide Mach-Zehnder interferometer 25 does not match the center frequency of the FSK signal light 15 _M , an AC component is generated in the control signal 17.
Therefore, the control signal 17 is sent to the optical path length control unit 27 of the waveguide Mach-Zehnder interferometer 25, and the optical path length control unit 27 controls so that the AC component becomes zero. The center frequency of the FSK signal light 15 _M can be made to coincide with the frequency at which the transmittance of 25 becomes the maximum point or the minimum point.

FIG. 3 is a diagram showing frequency characteristics of transmittance of the waveguide type Mach-Zehnder interferometer 25.

In the figure, the horizontal axis represents the frequency of incident light and the vertical axis represents the transmittance in percent. Also, the solid line and the dash-dotted line are
The frequency dependence of the light emitted from the two output ports a and b is shown, and the phases are different from each other by 180 °.

Here, the center frequency of the FSK signal light is controlled to a frequency at which the transmittance of the periodic filter (waveguide type Mach-Zehnder interferometer 25) becomes maximum (minimum), and the frequency shift is set to half of the transmittance cycle. In this case, as shown by the thick solid line arrow in FIG. 3, the deviation occurs at frequencies f _{11 to} f ₁₂ at which the transmittance becomes 50%. When the frequency deviation of the FSK signal light fluctuates and becomes longer than half of the transmittance cycle (thin solid arrow) or becomes shorter (dotted arrow), the FSK signal light has frequencies f _{21 to} f ₂₂ or Deviation occurs at frequencies f ₃₁ and f ₃₂ .

FIG. 4 shows the detection outputs of the photodiodes 26 ₁ and 26 ₂ corresponding to the frequency deviation of the FSK signal light and its fluctuation,
FIG. 7 is a diagram showing, on a time axis, a control signal 17 obtained by taking the difference between the two.

FIG. 4A is a diagram showing a state of frequency shift of the FSK signal light.

FIGS. 4 (b) and 4 (c) show FS that shifts at frequencies f ₁₁ and f _12.
K signal light, each output port a waveguide type Mach-Zehnder interferometer 25, each of the photodiodes 26 ₁ corresponding to b, 26 ₂ of the detection output i _a, indicates the i _b, Fig. 4 (d) are the difference shows the _(i a -i _b) a is the control signal 17.

That is, there is no fluctuation in the frequency shift of the FSK signal light, and the predetermined shift amount (frequency shift is half the transmittance cycle (2 GHz))
, The level of the light emitted from the two output ports a and b of the waveguide type Mach-Zehnder interferometer 25 (α)
Are the same, and the control signal 17 is not output (output 0).

FIGS. 4 (e) and 4 (f) show FS that shifts at frequencies f ₂₁ and f _22.
The K signal light causes the detection outputs (thin solid lines) of the photodiodes 26 ₁ and 26 ₂ corresponding to the output ports a and b to have frequencies f _{11 to} f.
Compared to the case of deviation at ₁₂ (thick solid line), FIG. 4 (g) shows the control signal 17 which is the difference.

That is, when the frequency shift of the FSK signal light has fluctuation exceeding a predetermined shift amount, the level difference (α + Δi /) of the light emitted from the two output ports a and b of the waveguide type Mach-Zehnder interferometer 25. 2, forward current + Δ due to α-Δi / 2)
i occurs, and the control signal 17 becomes a DC signal with a normal voltage.

FIGS. 4 (h) and 4 (i) show FS that shifts at frequencies f ₃₁ and f _32.
The K signal light detects the detection outputs (thin solid lines) of the photodiodes 26 ₁ and 26 ₂ corresponding to the output ports a and b at frequencies f ₁₁ and f, respectively.
Compared to the case of deviation at ₁₂ (thick solid line), FIG. 4 (j) shows the control signal 17 which is the difference.

That is, when there is fluctuation below the predetermined deviation in the frequency deviation of the FSK signal light, the reverse current −Δ
i occurs, and the control signal 17 becomes a negative voltage DC signal.

In this way, the control signal 17 is obtained as a DC signal corresponding to the fluctuation of the frequency shift of the FSK signal light, and by feeding back this signal as the control signal of the laser diode 21,
It is possible to control the frequency shift of the K signal light.

In the present embodiment, via a changeover switch 28 for interlocking the control signal 17 to the optical frequency selection switch 24, captures the corresponding variable attenuator 20 _M, frequency polarized by controlling the amplitude of the digital signal 11 _M Although a configuration in which shift is controlled, optical frequency multiplexed by FSK signal light 15 ₁ to 15 _N center frequency spacing (10 GHz) is half of the transmittance period of the waveguide type Mach-Zehnder interferometer 25 (4 GHz) Since it is an odd multiple (5 times) of (2 GHz), that is, a half integer multiple, the shift direction of the frequency shift of each FSK signal light 15 ₁ to 15 _N and the sign of the control signal 17 are different for each FSK signal light. Invert. Therefore, it is necessary to successively reverse the attenuation characteristics for the variable attenuator 20 ₁ to 20 _N of the control signal 17.

In addition, by making the interval of the center frequency of each FSK signal light 15 ₁ to 15 _N coincident with an integer multiple of the transmittance period of the waveguide type Mach-Zehnder interferometer 25, all variable attenuators have exactly the same input signal. It is possible to have a damping characteristic.

By the way, in the present embodiment, the configuration example using the waveguide type Mach-Zehnder interferometer 25 as the periodic filter is shown.
For example, it is possible to use a Michelson interferometer or the like. Further, since the variable attenuators 20 _{1 to} 20 _N control the amplitudes of the digital signals 11 _{1 to} 11 _N according to the control signal 17, other devices having the same function, such as an automatic gain controller, can be used. It is also possible to use. Further, the configuration using the optical frequency selection switch 24 for selecting the FSK signal light 15 _M to be stabilized and controlled in the latter stage of the optical multiplexer 23 is shown, but one FSK signal before the optical multiplexer 23 is combined in the former stage of the optical multiplexer 23. A configuration using a switch that selectively branches light may be used.

FIG. 5 is a block diagram showing the configuration of the second embodiment of the optical FSK frequency deviation stabilizing circuit of the present invention.

In addition, the present embodiment is characterized in that the periodic filter is processed at the electric signal level. That is, using two intermediate frequency differential detectors, the transmittance period is set to 4 (GHz), and the center frequency interval of each optical frequency-multiplexed FSK signal light is stabilized to 10 (GHz). In the configuration, it is assumed that the frequency shift of each FSK signal light is stabilized to 2 (GHz) which is half of the transmittance cycle.

In FIG. 5, variable attenuators 20 _{1 to} 20 _N and adders 21 _{1 to} 21
_{The configurations of N 2} , laser diodes 22 _{1 to} 22 _N , optical multiplexer 23, optical frequency selection switch 24, changeover switch 28 and control circuit 29 are the same as in the first embodiment.

In this embodiment, the FSK signal light 15 _M (1 ≦ M ≦ N) of one wavelength selected by the optical frequency selection switch 24 is coupled by the coupler 51.
And is combined with the local light 31 emitted from the laser diode 52. The combined light emitted from the coupler 51 is received by each of the photodiodes 53 ₁ and 53 _2, and the intermediate frequency signals 33 _{1 to} 33 ₂ obtained as respective outputs are respectively passed through the intermediate frequency delay detectors 54 _{1 to} 54 ₂ . Is input to the differential amplifier 55, and the output thereof is used as the control signal 17 via the changeover switch 28 for switching between 1: N and the corresponding variable attenuator 20 _M.
Sent to.

Further, the intermediate frequency signal 33 ₂ output from the photodiode 53 ₂ is branched and input to the intermediate frequency discriminator 56, and the frequency difference signal 35 that is the output thereof is supplied to the bias current 37 of the laser diode 52 for local light. It is a configuration to be added.

Here, one of the intermediate frequency delay detectors 54 ₁ can have a transmittance having a frequency characteristic shown by a solid line a in FIG. 3 by using a divider or other branching element having no phase shift, and the other one. Intermediate frequency delay detector 54
_For No. ₂ , by using a branch element of 180 ° hybrid, the transmittance can have the frequency characteristic shown by the one-dot chain line b in FIG. Therefore, by inputting the outputs of the intermediate frequency delay detectors 54 ₁ and 54 ₂ to the differential amplifier 55, the same control signal 17 as that of the first embodiment is obtained according to the principle described with reference to FIG. be able to.

In addition, in this embodiment, each optical frequency multiplexed FS
The center frequency interval of the K signal light varies depending on the intermediate frequency delay detector 5
4 _1, so relative to ₅₄₂ of the transmission cycle is set to half-integer multiples as in the first embodiment, it is necessary to successively reverse the attenuation characteristics for the variable attenuator 20 ₁ to 20 _N of the control signal 17 .

Also, the output of the intermediate frequency discriminator 56 is
The difference between the center frequency of the signal light 15 _{M and} the center frequency of the local light 31 can be obtained. Therefore, the frequency difference signal 35
By controlling the laser diode 52 by using, the center frequency of the intermediate frequency signals 33 ₁ , 33 ₂ is made to coincide with the frequency at which the transmittance of each intermediate frequency delay detector 54 ₁ , 54 ₂ becomes the maximum point or the minimum point. Can be made.

In the present embodiment, the frequency difference signal 35 extracted from the intermediate frequency signal 33 ₂ is used to control the frequency of the local light 31.However, the intermediate frequency signal 33 _{1 is} extracted from the other intermediate frequency signal 33 ₁ or both. The same applies to the case of using the signal.

As described above, in this embodiment, the FSK signal light 15 ₂ is converted into an electric signal in the intermediate frequency band by the heterodyne detection, and the frequency shift thereof is a periodic filter constituted by the intermediate frequency delay detectors 54 ₁ and 54 _2. It is characterized by a structure that stabilizes in accordance with the transmittance cycle of.

It should be noted that in the configuration in which the electric signal in the baseband frequency band is converted by the homodyne detection, the intermediate frequency delay detector 5
4 _1, 54 by the same circuit configuration by simply adjusting the _second delay time constant, it is possible to obtain a control signal 17 to stabilize the frequency shift.

FIG. 6 is a block diagram showing the configuration of a third embodiment of the optical FSK frequency deviation stabilizing circuit of the present invention.

Incidentally, in the present embodiment, similarly to the first embodiment, a configuration using a waveguide type Mach-Zehnder interferometer as a periodic filter is shown, and the transmittance period of the waveguide type Mach-Zehnder interferometer,
The same applies to the center frequency interval of each optical frequency-multiplexed FSK signal light and the stabilized frequency shift value of each FSK signal light.

Here, the feature of this embodiment is that each FSK signal light 1
Instead of stabilizing the frequency deviation of 5 ₁ to 15 _N by controlling the amplitude of the digital signals 11 _{1 to} 11 _N , a multi-electrode laser diode is used for each laser diode and the bias current is controlled. is there. That is, by controlling one bias current of the multi-electrode laser diode, the frequency modulation efficiency is changed to stabilize the frequency shift.

That is, in FIG. 6, the multi-electrode laser diode 61
_1-61 the one electrode B of _N, digital signal 11 ₁ to 11 _N
And the bias currents 41 _{1 to} 41 _N are added by the adders 63 _{1 to} 63 _N and injected, and the control signal 17 and the bias currents 43 ₁ to 43 _N are added to the other electrode A by the adders 65 _{1 to} 65 _{N. The} configuration is such that _{N is} added and injected. Therefore, in addition to eliminating the need for variable attenuators 20 ₁ to 20 _N of the first embodiment shown in FIG. 2, the optical multiplexer 2
3, optical frequency selection switch 24, waveguide type Mach-Zehnder interferometer 25, photodiodes 26 ₁ and 26 ₂ , changeover switch 28
The control circuit 29 has the same configuration.

Here, regarding the control example of the frequency modulation efficiency in the distributed feedback (DFB) type laser diode having three electrodes,
This will be described with reference to FIG.

FIG. 7A shows an electrode A, an electrode B, and an electrode B in order from the light emitting end side.
A distributed feedback laser diode having an electrode C is shown, in which a transmission signal (digital signal) is added to the bias current and injected into the electrode C.

In Fig. 7 (b), 40 (m
The frequency deviation of the light emitted when the bias current of (A) is injected and the bias current of 10 (mA) or 20 (mA) is injected into the electrode A is shown by a solid line and a dashed line, respectively.
The horizontal axis is the frequency of the transmission signal (digital signal) (MH
z), and the vertical axis represents frequency deviation (1 scale is 2 dB).

In this way, when the current amplitudes of the transmission signals are equal, the frequency shift of the laser diode changes in accordance with the change of the bias current injected into the electrode A, so the frequency modulation efficiency of the laser diode changes. Can be said.

FIG. 8 is a diagram showing changes in the oscillation wavelength when the injection current value of the electrode A closest to the light emitting end side of the three-electrode distributed feedback laser diode is changed.

As shown in the figure, the injection current of electrode A is 10 (mA) to 20
It can be seen that even if the value is changed to (mA), the change in the oscillation wavelength is minute.

In the multi-electrode distributed feedback laser diode, the frequency modulation efficiency and the change in the oscillation wavelength according to the change in the bias current are described in, for example, the document “Y.YOSHIKUNI and G.MO”.
TOSUGI, “Multielectrode Distributed Feedback Laser
for Pure Frequency Modulation and Chirping Supres
sed Amplituted Modulation ", J.Lightwave technol., Vo
I.LT-5, No. 4, 1987, pp, 516-522 ”.

Thus, in this embodiment, the control signal 17 is set to the bias current 43
_By adding it to _M and injecting it into the electrode A of the multi-electrode laser diode 61 _M , the FSK signal light 15 as in the first embodiment is obtained.
The frequency shift of _M can be controlled (stabilized).

In addition, also in this embodiment, each optical frequency multiplexed FS
The center frequency interval of the K signal lights 15 ₁ to 15 _N is set to a half integer multiple with respect to the transmittance period of the waveguide type Mach-Zehnder interferometer 25, as in the first embodiment. Therefore, each adder 65 ₁
~ 65 _N is the control characteristic of the frequency deviation with respect to the control signal 17,
That is, it is necessary to sequentially reverse the change characteristics of the bias current injected into the electrodes A of the multi-electrode laser diodes 61 _{1 to} 61 _N. Also, by making the center frequency interval of each FSK signal light 15 ₁ to 15 _N coincident with an integral multiple of the transmittance period of the waveguide type Mach-Zehnder interferometer 25, an adder that gives the same bias current change to the control signal 17 Can be used.

Although it is possible to control the frequency shift of the FSK signal light by controlling the bias current, the single electrode laser diode can be used. The center frequency of light also changes at the same time. Therefore, in this case, the waveguide type Mach-Zehnder interferometer is adjusted according to the change of the center frequency.
It is necessary to make the optical path length control unit 27 of 25 follow at high speed, and a complicated control system and demodulation system are required. In the multi-electrode laser diode, as described above, by appropriately controlling the bias current injected into each electrode, it is possible to change only the frequency shift without changing the center frequency of the FSK signal light.

By the way, the second embodiment is characterized in that a periodic filter (intermediate frequency delay detectors 54 ₁ and 54 ₂ ) that processes at the electric signal level is used in the configuration of the first embodiment. Also in the configuration of the third embodiment, the periodic filter can have the same configuration as that of the second embodiment.

〔The invention's effect〕

As described above, the present invention matches the center frequency intervals of a plurality of FSK signal lights with an integer multiple or a half integer multiple of the transmittance cycle of the periodic filter, and corresponds to a half cycle of the transmittance cycle of the periodic filter. With the configuration that controls the frequency deviation of each FSK signal light within the frequency range to be used, it is possible to stabilize the frequency deviation with one periodic filter and control means for each of a plurality of FSK signal lights. .

That is, it is possible to realize a frequency shift stabilization circuit for a plurality of FSK signal lights with a simple configuration, and it is possible to easily construct a highly reliable optical frequency multiplex transmission system.

[Brief description of drawings]

FIG. 1 is a block diagram showing the basic configuration of the present invention. FIG. 2 is a block diagram showing the configuration of the first embodiment of the present invention. FIG. 3 is a diagram showing a frequency characteristic of transmittance of a waveguide type Mach-Zehnder interferometer. FIG. 4 is a diagram explaining the principle of detecting fluctuations in the frequency shift of the FSK signal light. FIG. 5 is a block diagram showing the configuration of the second embodiment of the present invention. FIG. 6 is a block diagram showing the configuration of a third embodiment of the present invention. FIG. 7 is a diagram for explaining an example of controlling the frequency modulation efficiency of a three-electrode distributed feedback laser diode. FIG. 8 is a diagram showing changes in the oscillation wavelength of a three-electrode distributed feedback laser diode. FIG. 9 is a diagram showing a schematic configuration of an optical FSK modulator. 11 …… Digital signal, 13 …… Bias current, 15 …… FS
K signal light, 17 …… control signal, 20 …… variable attenuator, 21 ……
Adder, 22 ... Laser diode, 23 ... Optical multiplexer, 24
...... Optical frequency selection switch, 25 ...... Waveguide type Mach-Zehnder interferometer, 26 ...... Photodiode, 27 ...... Optical path length control unit, 28 ...... Changeover switch, 29 ...... Control circuit, 31 ...... Local light, 33 …… Intermediate frequency signal, 35 …… Frequency difference signal, 37 …… Bias current, 41, 43 …… Bias current, 51
...... Coupler, 52 …… Laser diode, 53 …… Photodiode, 54 …… Intermediate frequency delay detector, 55 …… Differential amplifier, 556 …… Intermediate frequency discriminator, 61 …… Multi-electrode laser diode, 63, 65 ... Adder, 91 ... Laser diode, 93 ... Bias current, 95 ... Digital signal, 97
…… Adder, 99 …… FSK signal light.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Hiroshi Toba 1-1-6 Uchiyuki-cho, Chiyoda-ku, Tokyo Nihon Telegraph and Telephone Corporation (56) Reference JP-A-58-56539 (JP, A) JP 61-134134 (JP, A)

Claims (1)

[Claims] 1. A plurality of light sources that shift a frequency according to a transmission signal and emit a plurality of FSK signal lights having different center frequencies, and a center frequency within a predetermined frequency range from the plurality of FSK signal lights. Selection means for selectively extracting a certain FSK signal light, the selected FSK signal light is input, a periodic filter that periodically changes the transmittance at a predetermined frequency interval, depending on the output of the periodic filter, A control means for matching the frequency with which the transmittance has a maximum value or a minimum value and the center frequency of the FSK signal light, and detecting a deviation of the frequency shift amount of the FSK signal light with respect to a half cycle of the frequency interval, An optical FSK frequency shift stabilization circuit, comprising: switching means for interlocking with the selection operation of the selection means and sending the output signal of the control means to the injection current control unit of the corresponding light source.